Water-borne adhesives have experienced increased use relative to other materials such as solvent-borne and hot melt adhesives. More specifically, solvent-borne adhesives have become disfavored due to heightened environmental regulation. Hot melt adhesives are typically energy intensive and tend to possess relatively low heat resistance properties. Moreover, the performance characteristics of the solvent-borne and hot melt adhesives are often limited by the low molecular weight of the polymers employed.
Nonetheless, the end properties of water-borne adhesives are often adversely affected due primarily to the high water level of the adhesive polymer latexes. Specific disadvantages relate to grain raising on wood products, curling of paper products, and overall slow setting speed of the adhesive. Accordingly, recent efforts have focused on producing water-borne adhesives having increased solids content. This has been done in an attempt to enhance parameters relating to speed of adhesive set, water resistance, and adhesion to difficult-to-bond substrates. U.S. Pat. No. 5,434,216 to Mudge proposes vinyl acetate-based polymer emulsion formed of various monomers and having a solids content which may range between 40 and 50 weight percent.
U.S. Pat. No. 4,921,898 to Lenney et al. proposes a vinyl acetate-ethylene copolymer emulsion having a solids content between 65 and 70 weight percent and utilizing 2 to 4 weight percent of polyvinyl alcohol.
U.S. Pat. No. 5,070,134 to Oyamada et al. proposes a vinyl acetate ethylene copolymer emulsion having a solids content between 65 and 75 weight percent. Oyamada et al. employs a nonionic surface active agent having an HLB of from 16.5 to 17.5 per 100 parts by weight of the vinyl acetate unit.
In spite of any potential advantages, the above disclosed emulsions are disadvantageous from the standpoint of processing time, in that it often takes at least four hours to form the polymerized emulsions from the starting materials. Accordingly, it would be desirable to obtain a copolymer emulsion having a high solids content and a lower viscosity which exhibits adequate end properties and may be polymerized in a reduced time period.